Flood Risk & Drainage Requirements for Arun District Council

Few planning authorities in the south east ask for as much drainage evidence before determination as Arun District Council. This page sets out Arun's flood risk and drainage requirements in full: the SuDS designer checklists published in October 2025, which expect winter groundwater monitoring, winter infiltration testing and construction-level drawings with the application itself, and the adopted Local Plan policies that back them. Unda prepares the flood risk assessments and surface water drainage strategies that get applications through this regime first time.

We prepare flood risk assessments for planning, property purchase, mortgage and insurance, and BREEAM across Arun district, alongside sequential and exception tests and drainage strategies. A flood risk assessment, a sequential or exception test and a drainage strategy are three different reports. They often travel together in an Arun planning submission, but they answer different questions: is the site safe, should the development be here, and where does the surface water go.

If you have a site anywhere between Pagham and Angmering and want to know what the council will ask of it, contact one of our experienced flood risk consultants on 01293 214444 or at enquiries@unda.co.uk, or start a quote and we will reply within 60 minutes.

If a drainage design in Arun district relies on infiltration, the council expects winter groundwater monitoring and winter BRE 365 testing to prove it will work. That evidence can only be collected between October and March, so a scheme that starts its ground investigation in April can lose the better part of a year.

Arun District Council's requirements at a glance

• The local planning authority. Arun District Council determines applications across the district outside the South Downs National Park, where the National Park Authority decides instead. Arun's own Engineering Services team reviews the drainage on every application of more than two units.
• The Lead Local Flood Authority. West Sussex County Council is the LLFA and the statutory consultee on surface water drainage for major applications. Read more on the role of LLFAs in planning.
• The flagship policies. Policy W DM2 (flood risk) and Policy W DM3 (sustainable urban drainage) of the Arun Local Plan 2011–2031, adopted 18 July 2018, set the tests every scheme must meet.
• The drainage evidence bar. Five SuDS designer checklists (October 2025) apply to minor and major applications alike. Items in bold on the outline checklist trigger an objection if they are missing.
• Discharge rates. Arun's checklists restrict surface water discharge to QBAR or 3 l/s/ha, whichever is greater, for all storms up to the 1% AEP event with climate change.
• The evidence base. A new Level 1 Strategic Flood Risk Assessment (JBA Consulting, March 2026) supersedes the 2016 version and finds most of the district at risk of groundwater emergence.
• The special zone. Every application in the Lidsey Wastewater Treatment Works catchment, however small, needs a Drainage Impact Assessment under Policy W DM1.

Who decides flood risk and drainage for an Arun planning application?

Four bodies share the decision, and applicants regularly write to the wrong one. Arun District Council is the local planning authority and makes the final call. West Sussex County Council, as Lead Local Flood Authority, advises on surface water drainage for major schemes. The Environment Agency covers main rivers and the coast. And uniquely in Sussex, a live internal drainage district still operates on the low ground beside the lower River Arun.

Southern Water sits outside the planning process but controls sewer connections, and Arun's checklists demand hydraulic capacity checks before any surface water connection to a public sewer is accepted. Arun District Council also enforces its own land drainage byelaws on ordinary watercourses.

Arun's October 2025 SuDS checklists: what you must submit up front

In October 2025 Arun published five SuDS designer checklists, aligned with the National Standards for SuDS (June 2025) and CIRIA's SuDS Manual: an outline checklist, a full/reserved matters checklist, a ground investigations checklist, a maintenance checklist and a verification checklist. They apply to minor and major applications alike, and the council is explicit that a failure to address the bold items will result in an objection. We covered the change when it landed in our article on Arun's new drainage checklists.

The practical effect is that work most councils accept at condition stage is expected before determination in Arun. The full checklist asks for site-specific construction detail drawings covering some twenty component types, from flow controls and catchpits to tree pits and green roofs, plus long sections, exceedance flow-route plans and demarcated catchments. Late drainage design is the single most common reason Arun applications stall, and it is avoidable. Our preliminary drainage design work exists for exactly this situation.

Winter groundwater monitoring, October to March

Arun expects groundwater monitoring in professionally installed boreholes within two metres of each proposed SuDS feature, read at least weekly, for a suggested six consecutive months between 1 October and 31 March. Smaller schemes get shorter windows: a roof area under 250 m² can monitor one point for two consecutive months between December and February, and under 20 m² one month is accepted. Desk studies are not accepted as evidence of groundwater levels, and if monitoring stops early because levels already rule out infiltration, the council assumes groundwater at ground level for buoyancy calculations. See our guide to groundwater monitoring requirements and the delays they cause.

Infiltration testing to BRE 365, in winter

Soakage testing must follow BRE Digest 365 or CIRIA R156 strictly: trial pits where the features are proposed, at the proposed base level, three tests in quick succession, filled to the design head and observed from full to empty. Borehole infiltration testing is not generally accepted, extrapolating an incomplete test is not accepted, and the council wants tests run in winter or spring. Our BRE 365 explainer and infiltration rate calculator cover the method, and this guide explains when testing is required at all.

Discharge rates, attenuation and interception

• Runoff destination. Designs must follow the six-step hierarchy: rainwater reuse, then infiltration, then a hybrid, then restricted discharge to a water body, a surface water sewer and, last, a combined sewer. Surface water never goes to the foul sewer, and gravity drainage is expected over pumps.
• If infiltration is unproven, show plan B. An outline application proposing infiltration without winter test data must set out an alternative discharge route. Where infiltration is the only option, winter ground investigation results are essential before determination.
• Discharge rate. Restricted to QBAR or 3 l/s/ha, whichever is greater, for all storms up to the 1% AEP event plus climate change. West Sussex County Council's 2018 LLFA policy works to the greenfield 1-in-1-year rate on major applications, so larger schemes need to satisfy both reviewers.
• Attenuation. No flooding of the system in the 1-in-30-year event and the 1-in-100-year event plus climate change contained on site without flooding buildings or blocking access. Half-drain times of 24 hours or less, 10% urban creep on residential roofs, and FEH22 rainfall inputs.
• Interception. The first 5 mm of rainfall should produce no runoff for the majority of events, in line with the national standards.
• The 1 m rule. One metre of unsaturated ground between the base of any infiltration feature and the expected peak groundwater level.

You can sense-check attenuation volumes and greenfield rates with our free drainage calculators before committing to a layout.

Where SuDS cannot go: the flood areas position statement

Arun also publishes a position statement barring SuDS from what it calls flood areas: fluvial Flood Zones 2, 3a and 3b and any 0.1% AEP surface water extent, unless the whole site sits within them or a listed exception applies, such as land modelled dry in the 1% AEP event with climate change, tidal-only zones or sealed tanks. SuDS are never accepted in flood storage or flood compensation areas. On a constrained coastal-plain site this can decide the layout before any architecture is drawn.

Groundwater: why Arun is this strict

The district sits on the Sussex coastal plain, where the superficial deposits are mostly poorly drained silty clays and a high water table is common, whether perched water, the aquifer or springs. The March 2026 Strategic Flood Risk Assessment finds the majority of the district at risk of groundwater emergence, with recorded events at Ford, Barnham and Aldingbourne. Groundwater appears on no national flood map, which is why the council leans so hard on measured, site-specific winter data.

High groundwater does more than break soakaways. It floats under-used attenuation tanks, infiltrates the foul sewer network, and keeps land wet for weeks after the rain has stopped. Every flood risk assessment we prepare in the district assesses groundwater alongside rivers, the sea, surface water and sewers, and our free geology and ground conditions map gives an early read on whether infiltration is even worth pursuing on your site.

Does Arun District Council require a flood risk assessment or drainage strategy?

More often than the national rules alone would suggest. Arun's validation requirements, in force since 17 October 2024, go beyond the standard Flood Zone 2 and 3 triggers: they call for a flood risk assessment where the Strategic Flood Risk Assessment shows more than a 25% risk of groundwater flooding, where a site falls in the future Flood Zone 3a, and for surface water drainage information on any scheme of two or more dwellings anywhere in the district. If the required documents are missing, the application is not validated. Our article on invalidated applications explains how that plays out in practice.

Not sure which rows catch your site? Check it on our free flood risk map, or read when a flood risk assessment is required.

Arun's Local Plan flood and drainage policies

The Arun Local Plan 2011–2031, adopted on 18 July 2018, deals with water in Chapter 18. Its policies are unusually specific: W DM3 writes winter groundwater monitoring and BRE 365 testing directly into development plan policy, which is why the council can hold the line so firmly at appeal. A new local plan is being prepared, with draft policies consulted on from 2026, but until adoption (currently timetabled for 2028) these policies decide applications.

Policy W DM2: what Arun's flood risk policy requires

Policy W DM2 permits development in areas at risk of flooding only where the sequential test is met and a site-specific flood risk assessment demonstrates the development will be safe, including access and egress, without increasing flood risk elsewhere, and will reduce flood risk overall. It also asks for flood warning and evacuation plans and for site drainage designed for exceedance events. That final pair catches people out: a technically safe building with no evacuation route, or no thought given to where an extreme storm overflows, still conflicts with the policy. Reports must apply the national climate change allowances and take account of the Lower Tidal River Arun Strategy where relevant.

Policy W DM3: what Arun's drainage policy requires

Policy W DM3 requires every development, major and minor, to identify SuDS opportunities at an early design stage, incorporate source control in private areas, follow the drainage hierarchy, and back any infiltration proposal with up to six months of winter groundwater monitoring and BRE 365 winter percolation testing. Systems must show no flooding in the 1-in-30-year event, contain the 1-in-100-year event plus a 30% climate change allowance without flooding property or highways, and be maintained in perpetuity through a funded maintenance and management plan agreed with the council. The October 2025 checklists are, in effect, W DM3 turned into a document-by-document demand list.

The Lidsey catchment: where every application needs a drainage impact assessment

The Lidsey Wastewater Treatment Works catchment, covering Barnham, Eastergate, Westergate, Yapton, Walberton, Aldingbourne and part of Middleton-on-Sea, has a long history of foul flooding caused by groundwater infiltrating the sewers and surface water inundating the foul network. Policy W DM1 responds by requiring a full Drainage Impact Assessment for major development in the catchment, and because the problem is cumulative, a Drainage Impact Assessment for all development, including minor schemes, covering foul disposal, flood storage capacity and surface water flows. Arun's Engineering Services are consulted on every application in the catchment, however small. If your site drains towards Lidsey, budget for this from day one.

The March 2026 SFRA and the sequential test

Arun's Level 1 Strategic Flood Risk Assessment, prepared by JBA Consulting and finalised in March 2026, replaces the 2016 evidence base. It defines the functional floodplain, Flood Zone 3b, as the 3.3% AEP event, and where no detailed modelling exists it treats Flood Zone 3a as a conservative stand-in, to be refined by a site-specific assessment. It confirms no critical drainage areas are currently designated in the district, maps groundwater emergence using JBA's dataset, and draws on detailed models including the 2024 Arun and Adur coastal model.

One wrinkle matters for sequential tests right now: the Environment Agency published new surface water climate change extents on the Flood Map for Planning on 28 May 2026, after the SFRA was finalised, and the council has said the latest data must be used. A sequential or exception test for an Arun site needs to reconcile the two, and the September 2025 planning practice guidance update means surface water now counts in the test as well. Where mapping is not good enough to settle the question, flood modelling can.

How Arun behaves in practice: the Angmering appeal

If you want proof the council means it, read the appeal decisions for the former Shrublands Nursery site at Roundstone Lane, Angmering, issued on 31 March 2025. A developer built 40 houses without discharging the pre-commencement drainage conditions; 27 were sold and occupied. The inspector found the soakage testing did not comply with BRE 365, that averaging results from distant trial pits was inappropriate, and that winter groundwater levels and the one-metre separation could not be confirmed without long-term monitoring. The drainage appeal was dismissed for conflict with policies W SP1, W DM1, W DM2 and W DM3, and because the undischarged conditions went to the heart of the permission, the permission was held never to have been lawfully implemented, with 40 completed homes standing on site.

The lesson is the one Arun's checklists spell out: drainage evidence comes first, not after commencement. It is also why an LLFA or engineering objection in this district is best pre-empted rather than answered, and why we front-load winter monitoring in every Arun programme we plan. When conditions do need discharging, our discharge of drainage conditions service handles the detailed stage.

Water neutrality in the north of the district

Parts of northern Arun fall within the Sussex North Water Supply Zone, where between September 2021 and late 2025 development had to demonstrate water neutrality to protect the Arun Valley wetlands. Natural England withdrew its position statement on 31 October 2025 after an agreement to cap the Pulborough abstraction licence, so water neutrality no longer needs to be demonstrated, though the 110 litres per person per day efficiency standard in Policy W DM1 still applies. The full story is in our water neutrality guide.

How national policy applies to an Arun application

Everything above sits on top of the national framework, not instead of it. The National Planning Policy Framework directs development away from flood risk through the sequential approach, and its practice guidance sets the vulnerability classes and the climate change allowances an Arun flood risk assessment must apply. The National Standards for SuDS, published on 19 June 2025, are the benchmark Arun's checklists are built around, including the interception target and the four SuDS pillars. Our NPPF guidance service covers the framework in detail, and our note on what drainage information planning permission needs explains strategy versus detailed design. The short version for Arun: the council asks for the detailed end of the national range, early.

Flood risk in Arun's area, briefly

The requirements make more sense against the geography. The tidal River Arun runs through the district to the sea at Littlehampton, tidal for some 40 km inland; the Aldingbourne, Lidsey and Ferring rifes drain a low, flat coastal plain that struggles to shed surface water; the coast at Bognor Regis, Felpham and Pagham takes the full force of southerly storms; and groundwater rises through the coastal plain's silty clays most winters. Storm Ciarán in November 2023 flooded Bognor, Shripney and Yapton, and Storm Kathleen in April 2024 prompted a formal flood investigation at Littlehampton. The £22 million Arun East Bank scheme, completed in 2015, now protects Littlehampton town centre from tidal flooding.

This page covers what the council requires. For the fuller local flood-risk picture, see our pages on flood risk assessment in Arundel, Littlehampton, Pagham and across West Sussex.

How Unda helps with flood risk and drainage for Arun planning

We know this authority's regime in detail: the checklists, the monitoring windows, the Lidsey rules, the policies the council cites at appeal and the way its Engineering Services team reads a drainage submission. We have prepared flood risk assessments and drainage strategies across Arun district and the wider West Sussex coastal plain, and we plan ground investigations around the October-to-March window so the winter data the council demands is there when the application goes in.

• Programme first. We tell you at the outset whether your site needs winter monitoring, how long it will take, and what can be submitted in the meantime.
• Checklist-complete submissions. Reports structured against Arun's own checklists and the national standards, so the bold items are answered before anyone objects.
• Both reviewers satisfied. Drainage designed to meet Arun's QBAR or 3 l/s/ha rule and the county council's greenfield expectations on major schemes.
• Trouble-shooting. Objections answered, conditions discharged and refused schemes re-worked, with the Angmering lessons in mind.

Contact one of our experienced flood risk consultants on 01293 214444 or enquiries@unda.co.uk to talk an Arun site through.

Flood risk and drainage services for every situation in Arun's area

• Flood risk assessment for planning. The site-specific assessment Policy W DM2 and the validation list call for.
• Surface water drainage strategy (SuDS). The drainage design and evidence Policy W DM3 and the October 2025 checklists demand.
• Sequential and exception tests. The locational case for developing where flood risk exists.
• Property purchase flood risk survey. For buyers and lenders weighing a home or site on the coastal plain.
• Flood risk assessment for insurance. Where premiums have risen or cover has been restricted.
• BREEAM flood risk assessment. Structured around the Pol 03 credits.
• Groundwater assessment. Assessed within every FRA, and in depth where the coastal plain water table drives the design.
• Flood modelling. Where mapping cannot settle levels, extents or the sequential case.

Flood risk and drainage contacts for Arun district

• Arun District Council Engineering Services. The drainage reviewer; the council asks designers to make contact before funding monitoring or testing. See the surface water drainage for planning hub.
• West Sussex County Council. The Lead Local Flood Authority, consulted on major applications and consenting ordinary watercourse works.
• Environment Agency. Main rivers, the coast, and the River Arun internal drainage district, where it acts as the drainage board. Floodline: 0345 988 1188.
• Southern Water. Sewer connection applications and capacity enquiries for foul and surface water.
• Planning policy. The adopted Arun Local Plan, the strategic flood risk assessment and the validation requirements on the council's site, with flood zones on the Flood Map for Planning.

Arun District Council flood risk and drainage requirements: frequently asked questions